We propose to measure the l3C/12C ratio in highly-purified DNA isolated from human autopsy cerebellar tissue specimens, for the purpose of estimating the rate of turnover of DNA in cerebellar granular neurons. These neurons constitute about 90% of the total population of cerebellar cells. There is substantial evidence that 13C/12C ratios of tissue constituents, including cerebellar DNA, are distinctly different in native-born Europeans and Americans. It is believed that this difference is caused by different photosynthetic pathways at the sources of European and American food chains. Thus, measurements of 13C/12C in DNA from cerebella of European-born immigrants to the United States should indicate the average rate of turnover of DNA over lifetime, but more work is required to refine the study.